Learning a Unix-like operating system is like a Herculean task for most of us. Most people even those working in the IT industry break into cold sweats on hearing words like Unix, Linux and Debian. Such aura of mystery and wizardry commonly associated with it is perhaps much to do with fear is unknown rather than the difficulty of learning the operating system(s).
One of the important advantage of learning a Unix-like operating system is in its transparency. If you want to understanding the intrusive details of how an operating system works you can dig right into the bowls of the operating system and to its source code. A closed source Operating system like Microsoft Windows NT XP or Vista will not allow you do that thus a very bad choice for learning OS concepts.
The boot process starts the moment you power up your PC and until the time the computer is able allow you to log on. Understanding this booting process of a GNU/ Debian Linux is an essential part of mastering the OS, the From Power Up To Bash Prompt HOWTO written a decade ago provides a detailed look at the boot process from Power up to the bash prompt . Even the experienced Linux users will learn from this HOWTO.
The author(s) of the HOWTO document sums it up as thus.
I find it frustrating that many things happen inside my Linux machine that I do not understand. If, like me, you want to really understand your system rather than just knowing how to use it, this document should be a good place to start. This kind of background knowledge is also needed if you want to be a top notch Linux problem solver.
This HOWTO is can be found in doc-linux package on GNU/Debian in text and html formats.The Debian Documentation page has an extensive list of manuals that are available to aid the learning the OS. Also visit the The Linux Documentation Project(TLDP) for more such HOWTO's , Guides and FAQ's.